This revised PPG competing renewal application is driven by the fundamental and therapeutic importance of costimulatory pathways for regulating T cell activation and tolerance, and builds upon our significant progress since initial awarding of this PPG in 2003. The synergy in our PPG is highlighted by 111 publications, 49 of which have been co-authored by multiple PPG investigators, and include 33 new publications since the first (May 2007) submission. Our PPG also has had a significant role in mentoring fellows who work on joint projects, and fostering development of junior faculty. Our PPG has not only provided novel insights to the field, but allowed sharing of novel mAbs and fusion proteins, and mouse strains with the broader scientific community that resulted in better understanding of the functions of costimulatory pathways above and beyond our PPG aims. The overall objective of our PPG is to achieve a mechanistic understanding of the roles of pathways in the B7:CD28 family in regulating T cell activation and tolerance. Building upon our published and preliminary data, these goals include: 1) investigate roles of these pathways in regulating T cell responses in target tissues versus lymphoid organs, 2) dissect functional hierarchy, dominance and redundancy among these pathways, and 3) study how these pathways regulate the balance between protective and pathogenic T cell responses, ultimately determining the fate of the immune response, using experimental models of transplantation, autoimmunity, and infection. The proposed PPG renewal application will provide a means by which PPG investigators can continue to work together to address these important issues and develop a comprehensive understanding of the functions of B7:CD28 family members, ultimately leading to development of novel immunotherapeutic strategies. This PPG will facilitate communication among PPG investigators and sharing of a rich collection of tools, transgenic/knockout mice and mAbs and Ig fusion proteins, to address these issues. The use of the same standardized reagents and experimental animals makes it possible to compare and contrast results in different microenvironments and disease models. There will be 3 Projects (Transplantation, Autoimmunity, Infection) and 3 Cores (Administrative, Antibody/lg fusion protein, Transgenic/Knockout). The "Administrative Core" will be responsible for providing scientific direction and coordination, fiscal oversight and administrative support for the PPG. Overall, this PPG should provide fundamental knowledge for therapeutic manipulation of these important regulatory pathways. PROJECT 1: T Cell Costimulatory Pathways: Functions and Interactions (Arlene H. Sharpe, Project Leader) PROJECT 1 DESCRIPTION (provided by applicant): We, and others, have shown that the negative T cell costimulatory pathways play a critical role in regulating alloimmune responses and tolerance. Our overall hypothesis is that the integration of positive and negative signals delivered by allogeneic hematopoietic and/or parenchymal cells under inflammatory conditions dictates tolerance vs tissue destruction. Our main goal is to define the relationship between these signals at the tissue level and outcome of alloantigen exposure during graft-vs-host disease (GVHD) and organ grafting models. Lessons learned from these distinct but interrelated and complementary models will further our understanding of the mechanisms of GVHD and organ allograft rejection and tolerance. These studies will lead to development of novel therapeutic strategies by harnessing the physiologic mechanisms that regulate immune responses. We propose to address three questions: 1) What is the role of hematopoietic vs parenchyma cell expression of negative costimulatory molecules in alloimmunity and tolerance? 2) What are the important and unique interactions between positive and negative costimulatory pathways that determine the fate of alloimmune response in an inflammatory environment? 3) What are the effects and mechanisms of targeting of these pathways in preclinical models of alloimmune responses? Our specific aims are: AIM 1: To investigate the functions of the PD-1:PD-L1,-L2 negative costimulatory pathway in GVHD and solid organ transplants. 1A. Study the role of the PD-1 pathway in GVHD focusing on the parenchymal vs hematopoietic cell expression of PD-1 ligands in regulating alloimmunity. 1B. Study the role of PD-1 pathway interactions with the CD28/B7 positive T cell costimulatory pathway that augments and donor Tregs that suppress GVHD. 1C: To investigate the functions and mechanisms of PD-1:PD-L1 and B7-1/PD-L1 interactions in alloimmune responses in models of solid organ transplantation. AIM 2: To investigate the functions of B7-H3 expression on hematopoietic vs parenchymal cells during GVHD (2A). In order to define the exact functions of this novel pathway in alloimmunity we plan to characterize the B7-H3 receptor, TLT2, and clone a putative second and inhibitory B7-H3 receptor (2B). We have a number of unique tools that will enable us to dissect the functions, mechanisms, and interactions of negative and positive costimulatory pathways and the microenvironment that will permit the optimal targeting of costimulatory pathways to inhibit detrimental T cell alloresponses that limit hematopoietic and solid organ allograft acceptance.